Tools > Options > Main > Startup, Leeza.

Firefox3 is looking good so far. Since beta 2 they've improved Javascript 1.8 support (see <a href="https://bugzilla.mozilla.org/attachment.cgi?id=273497">here</a>) and significantly ironed stuff out with the SVG image format (see <a href="http://www.croczilla.com/svg/samples/svgtetris/svgtetris.svg">here</a>, <a href="http://files.myopera.com/orinoco/svg/USStates.svg">here</a>, <a href="http://starkravingfinkle.org/projects/demo/svg-bubblemenu-in-html.xml">here</a> and <a href="http://www.amaltas.org/svgapp/">here</a> - these are amazing). They have also smoothed out some bugs in AJAX support (see <a href="http://developer.mozilla.org/presentations/eich-ajax-experience-2007/">here</a>), but I can't find the page I really wanted to test AJAX on - one where you could drag photographs around and resize/rotate them.

EDIT: Found it in my IRC logs of all places.
<a href="http://developer.mozilla.org/presentations/eich-ajax-experience-2007/photos.svg">AJAX stuff</a>. Looks like it's still broken, unfortunately, 'cause that is one of the best examples. It renders correctly in Firefox2, though, I believe.
EDIT2: Looks like <a href="http://people.mozilla.com/~vladimir/demos/photos.svg">Vladimir fixed it</a>, but didn't deploy the fixed script.

I've still come across a bug with certain WYSIWIG in-browser editors coded in Javascript though. Also, the aggressive memory management in Firefox3 is great.